1. Field of the Invention
The present invention relates to torque coupling assemblies in general, and more particularly to a torque coupling assembly including an electromagnetic actuator controlling a variable pressure-control valve.
2. Description of the Prior Art
Hydraulic couplings are used in various vehicular drivetrain applications to limit slip and transfer drive torque between a pair of rotary members. In all-wheel drive applications, hydraulic couplings are used to automatically control the drive torque transferred from a driven member to a non-driven member in response to speed differentiation therebetween. In limited slip applications, couplings are used in association with a differential to automatically limit slip and bias the torque distribution between a pair of rotary members.
Such hydraulic couplings conventionally use a friction clutch between the rotary members as a limited-slip device. The friction clutch may be selectively actuated by various hydraulic actuator assemblies. The hydraulic actuator assemblies internal to a torque-coupling case often include displacement pumps disposed inside the torque-coupling case and actuated in response to a relative rotation between the torque-coupling case and the output shaft. The displacement pumps are usually in the form of internal gear pumps, such as gerotor pumps adapted to convert rotational work to hydraulic work. In the internal gear pumps, an inner gear having outwardly directed teeth cooperates with an external gear having inwardly directed teeth so that fluid chambers therebetween increase and decrease in volume as the inner and outer gears rotate in a housing.
Pump type hydraulic couplings, such as active limited slip differentials, employ the internal pump to convert the spin speed difference between the one of the output shafts and the differential case to a hydraulic pressure that actuates a piston (hydraulic cylinder), which in turn activates a multi-plate clutch pack. In addition, an electromagnet-activated pressure-relief throttle valve, disposed at a fluid outlet hole of the pump, controls the fluid pressure generated by the pump and thus the torque level of the limited slip device. Prior-art pump type active limited slip differentials are acceptable for low-speed mobility situations (e.g. split-μ hill climb), but they lose its controllability for medium-to-high-speed handling maneuvers. This failure of the pump-type coupling is caused by the fact that fluid inlet and outlet holes positioned at significantly high outer radius of the differential case, resulting in the centrifugal loss/draining of the fluid from the pump and the piston when the differential case is spinning.
The reason for this is that the present pump-type active limited slip differentials employ an annular electromagnet that is oriented upright, i.e. open at its outer radius. An annular armature is disposed at the outer radius of the electromagnet with a small amount of axial position offset. The energized electromagnet axially pulls the armature towards the differential case, choking the pressure-control valve disposed at the outlet hole of the differential case. Such a radial arrangement of the electromagnet and armature in the prior art pump type active limited slip differentials renders no choice but to position the fluid inlet and outlet holes at the radial position equal to or larger than the radius of the armature, which is usually larger than the diameter of the hydraulic pump and the piston. As a result, when the differential case spins in response to the vehicle speed, the hydraulic fluid in the pump and the piston is centrifugally drained through the oil inlet and outlet holes, resulting in the failure of the differential system in terms of time delay and abrupt engagement of the clutch. Therefore, the prior-art pump type active limited slip differentials fail to work for medium-to-high-speed handling maneuvers.
Another disadvantage of the prior-art active limited slip differentials is their use of the pressure-relief throttle valve in the form of a compression-type ball valve or cone valve that requires relatively high actuation force and suffers from a “pressure defeat problem”. The “pressure defeat problem” denotes that static pressure and hydrodynamic force open the pressure-relief throttle valve against the electromagnetic force. As illustrated in FIG. 1, an output torque of the torque-coupling assembly of the prior art first increases in a Linear Control Regime as speed differential between the torque-coupling case and the output shaft increases. Then, in a Pressure Defeat regime, the output torque decreases (thus losing torque capacity) as the compression-type ball (or cone) valve is defeated by the rising hydraulic pressure generated by the pump. Finally, the output torque increases again in a Pure Orifice Regime when the compression-type ball valve is fully open (defeated).
Thus, while known hydraulic couplings, including but not limited to those discussed above, have proven to be acceptable for some vehicular driveline applications and conditions, such devices are nevertheless unacceptable for some operational conditions and susceptible to improvements that may enhance their performance and cost. With this in mind, a need exists to develop improved hydraulic torque-coupling assemblies that advance the art.